1. Field of the Invention
This invention relates to a reamer for shaping bone sockets which has improved cutting characteristics. More particularly, the invention relates to a hemispherical reamer for shaping acetabulum.
2. Description of the Prior Art
Power driven reamers or bone cutters are utilized to round out and reshape the acetabular cavity or socket of the hip, following the destruction of cartilage or bone at the hip socket. Such power driven bone cutters utilize a plurality of blades having the cutting edges projecting slightly from the rotary hemispherical head of the cutter. In U.S. Pat. No. 3,633,583 to Meyer Fishbein dated Jan. 11, 1972, there is shown a substantially hemispherical head in which a single surgical blade is so shaped as to provide two radially disposed cutting edges on opposite sides of the rotational center line of the head. The edges project just slightly above the adjustment substantially hemispherical surface of the cutting head and the cutting edges are beveled in opposite directions on opposite sides of the axis of rotation so that both edges will cut during rotation of the head. Troughs or grooves are formed in the head forward of the cutting edges to transport the cut material from the surface of the head to the back end of the head. U.S. Pat. No. 4,621,637, also to Meyer Fishbein, relates to a hollow acetabular reamer having three radial cutting blades. The blades have slots adjacent thereto to allow debris to fall within the cup.
An additional bone cutter is disclosed in U.S. Pat. No. 4,131,116 which relates to a hemispherical shaped cutter head with a plurality of radial cutter slots passing from the exterior surface of the cutter head into a hollow chamber within the head. At the trailing side of each slot is formed a cutting edge which projects slightly above the surface of the cutter head. Each cutter edge extending beyond the surface of the head removes a small portion of bone or cartilage from the hip socket and this bone passes through the slot forward of the cutting edge into the head chamber where it is retained until the head is removed from the drive shaft. Thus, with this type of cutter head there is no loose bone or cartilage in the vicinity of the hip socket during rotation of the cutter head. By using at least three slots and cutting edges, the head is equally loaded during rotation in all radial directions so that there is no tendency for tipping of the axis of rotation of the head as the cutting operation proceeds. The cutting edges and slots of U.S. Pat. No. 4,131,116 are so formed in the head that at least one slot and cutting edge passes over the point at which the end of the axis of rotation intersects the cutter head so that the complete surface of the hip socket is removed. However, in cases where it is desired to leave a flat portion at the bottom of the socket, all of the slots and cutting edges will terminate short of the point at which the axis of rotation intersects the head.
The present invention is an improved version of the cutting head of U.S. Pat. No. 4,131,116. The cutting head of the present invention has superior cutting ability by providing a reamer having a cutting head containing helical slots and cutting edges which move the scrapings and bone cuttings into a normally sealed hollow chamber in the cutter head during rotation of the head. Thus, the cuttings cannot escape into the operating area until such time as the power drive is stopped and the head is removed for cleaning. Furthermore, by the use of at least three helical cutting blades, the maintenance of the axis of cutting is more easily assured and by providing a quick disconnect of the cutter head from the drive shaft, it is possible to easily change the size of the cutter head to enlarge the size of the hip socket.
The use of helical slots produces more efficient cutting in that bone or tissue is sliced rather than scraped off, as is the case with radial slots. The proper rake cutting angle and helical slot angle produce lower temperature cutting and can be varied to cut more or less aggressively. A positive rake angle is used to ensure that the cutting edges are pulled into the material to be cut.